How the heck do you know what car you can use in Career mode?

[ManteoMax]

FAQ pinned at the top of the forum:

@ManteoMax

FM6 FAQ: Stories of Motorsport

Forums: Events List thread - with list of eligible and non-eligible cars for each Volume
External: ManteoMax.com - FM6 sortable spreadsheet of cars and Divisions, and Career events list
External: Kudosprime.com - FM6 sortable database of cars and Divisions with photos of each car

From the Help menu description:
“The Stories of Motorsport spans five Volumes, starting in Super Street and ending in Ultimate Motorsport. Each Volume is its own career, a comprehensive experience that lets the player travel the world and celebrate automotive history, both past and present. Each volume is broken into three distinct Series. Think of these as chapters that range from tours of iconic racing venues to the ultimate collection of wet weather races. Tackle each Series with your choice of six unique Car Divisions. Each Car Division is a selection of cars that speaks to the essence of the Volume in which they reside.” (s) | (d)

The main career mode in Forza Motorsport 6 guides the player through a collection of cars and circuit tracks with increasing length and difficulty. As you progress through Stories of Motorsport, you will be given the opportunity to enter Showcase events and then return to Stories of Motorsport.

In FM6 most of the cars are assigned a Division. These body types are used to select certain eligible cars in each Volume of the career. If a car is not assigned a Division or is not one of the options, it can’t be used in career but it can still be used in Free Play, Rivals, and Multiplayer. Each volume has three Series of six optional divisions. Once a player starts a series they must complete it or start over in a different division before unlocking the next division. However, players may continue to use the same division cars for the next series or choose a different division. One division in each series in a volume must be completed before the next volume is unlocked. Players must finish each race in 3rd place or higher in order to proceed to the next race.

Restrictions for each series are not shown but you can determine them by opening one, selecting Buy Cars, and seeing where each car PI is, or buy entering with a low PI stock car which will prompt you to upgrade, and selecting Manual Upgrade will show the PI limit. If you see an “Illegal Car” message you will have to remove upgrades on your car to reduce the PI to make it legal. Buying cars from this menu includes automatically selected upgrade parts to bring the car to the top of the restriction limit; alternately you can back out of the event and go to the main menu Buy Cars list, buy a car stock, and then upgrade it to your preference. Note that some series have a PI restriction lower than the maximum for that class, so some downloaded tuning setups won’t be eligible. Opponent Drivatars in Stories of Motorsport are upgraded to the top of the event restriction and do not adjust down based on the player’s chosen car, so using a stock car will be less competitive.

Class maximum PI thresholds: E300 | D400 | C500 | B600 | A700 | S800 | R900 | P998 | X999

Stories of Motorsport Volumes and Series restrictions

Click links or scroll down to see eligible cars for each Volume.

Volume 1 - Super Street

• Japanese Street Kings, class: C500
• Compact Sport Coupe, class: D400
• Hot Hatchbacks, class: D400
• Ultimate SpoCom, class: B600
• Rally Day Heroes, class: C500
• Classic Compact, class: E300
  Volume 2 - Sports Icons
• Ultimate Sport Machines, class: A660
• Modern Sport Legends, class: B600
• Executive Heavyweights, class: A650
• Track Toys, class: A700
• Sport Utility Juggernauts, class: C500
• Historic American Muscle, class: D400
  Volume 3 - Grand Touring
• Modern Hypercar, class: R880
• Sport GT, class: S750
• Exotic GT, class: S800
• Birth of the Supercar, class: S800
• Ultimate Grand Tourer, class: S750
• Vintage Touring, class: D400
  Volume 4 - Professional Racing
• Modern GT Racing, class: R865
• V8 Supercars, class: S800
• Early GT Racing, class: R850
• Global Touring Cars, class: S800
• Formula E, class: S750
• Historic Sportscar Racing, class: A675
  Volume 5 - Ultimate Motorsport
• Endurance Prototypes, class: P998
• Legends of Grand Prix, class: S775
• Can-Am, class: R855
• IndyCar, class: X999
• 1970s Grand Prix, class: R900
• Modern Grand Prix, class: X999